Mobile communication systems may be categorized into first generation systems employing analogue technology, second generation systems employing digital technology, third generation systems providing IMT-2000 high speed multimedia services, and fourth generation systems providing ultra high speed multimedia services.
FIG. 1 illustrates the architecture of a typical HRPD system.
Among third generation mobile communication systems, the CDMA HRPD (high rate packet data) system and WCDMA HSPA (high speed packet data) system support channel structures for high-speed data transmission. The HRPD system is based on code division multiple access (CDMA) technology and is illustrated in FIG. 1. The HRPD system includes a packet data service node (PDSN) 110 that is connected to the Internet and sends high speed packet data to base stations 130 and 131, and packet control functions (PCF) 120 and 121 for controlling the base stations 130 and 131. The base station 130 wirelessly communicates with multiple user equipments 140 and 141, and sends high speed packet data to a user equipment at the highest data rate.
FIG. 2 illustrates the architecture of a typical LTE system.
Fourth generation mobile communication systems, which have evolved from third generation systems like HRPD systems, aim for transmission speeds of 20 Mbps or more to provide ultra high speed multimedia services, and mainly use orthogonal frequencies based on orthogonal frequency division multiplexing (OFDM). A representative fourth generation mobile communication system is an LTE system or LTE-advanced system (having been standardized by 3GPP). Referring to FIG. 2, the LTE system includes base stations (eNBs) 220, 221 and 222 that wirelessly communicate with multiple user equipments 210 to provide ultra high speed multimedia services, MME/S-gateway (serving gateway) 230 and 231 that manages mobility, call processing and data transmission paths of user equipments, and a PDN gateway (packet data network gateway) 240 that is connected to the Internet 250 and sends high speed packet data via base stations to user equipments.
With recent advances in communication technology, various devices such as controllers, instruments and home appliances, which were not connected with communication systems in the past, are increasingly connected to wired or wireless communication systems. These devices connected with communication systems conduct metering or control operations without direct human intervention so as to increase efficiency and decrease maintenance costs.
Unlike conventional communication where humans are principal agents, communication occurring between devices (such as controllers, instruments and home appliances) and a communication system is referred to as machine-to-machine (M2M) communication. When M2M communication was first conceived for remote control or telematics in the early 1990s, the market was very narrow. Thereafter, M2M communication has rapidly advanced to such an extent as to create a worldwide market covering various fields including point-of-sale (POS) systems, fleet management for security applications, remote monitoring of machines and facilities, and smart metering of operating times or usage of heat or electricity of machines and facilities.
In the description, a user equipment (UE) or user terminal performing M2M communication is referred to as an M2M UE.
M2M UEs may have different characteristics in comparison to existing regular user equipments. Representative characteristics of M2M UEs are listed below.
1. M2M UEs such as controllers and measurement instruments do not move or move very infrequently.
2. Some M2M UEs send and receive data only in preset time durations.
3. Some M2M UEs may be tolerant of delay in data communication.
4. Some M2M UEs do not need a function for voice communication.
5. M2M UEs do not need paging from mobile communication systems, and may make attachment requests when data communication is necessary.
6. As various devices may support M2M communication, the number of M2M UEs may be much greater than that of regular UEs in regions where population density is high.
When a large number of M2M UEs make attempts for system access in a preset time duration owing to their characteristics described above, communication congestion that cannot be handled with congestion control schemes employed by existing mobile communication systems may be caused.
FIG. 3 is a sequence diagram illustrating an access procedure specified in an existing LTE system. A UE 210 having data or signaling data to be sent transmits a preamble (access request message) through system-specified access resources to a corresponding eNB 220 (310). Here, the preamble is selected from a number of preambles specified for the access procedure in consideration of channel states and the amount of data to be sent of the UE 210. Upon reception of the preamble, the eNB 220 sends a response message, which contains resource allocation information to be used for subsequent data transmission and a temporary identifier to be used for access, to the UE 210 (320). Upon reception of the response message, the UE 210 sends a control signal for connection establishment by use of resources indicated by the response message to the eNB 220 (330). When two different UEs make system access attempts using the same preamble at step 310, they may simultaneously make attempts for connection establishment at step 330 according to the message received at step 320. To resolve such contention, the eNB 220 sends a message containing a unique identifier of a selected UE to the UE 210 to thereby notify the UE 210 of selection (340).
In the access procedure of FIG. 3, when contention between different UEs frequently arises, the eNB 220 may configure access delay for UEs not selected at step 302. That is, when a UE receives a response message containing access delay information at step 320 (i.e., not selected by the eNB 220) after sending a preamble at step 310, the UE delays making a system access attempt for a given time. Here, the access delay time is randomly selected from a range indicated by the access delay information.